3.5.4. ContentFilteredTopic

A ContentFilteredTopic is a specialization of the broader concept of TopicDescription. A ContentFilteredTopic is a Topic with filtering properties. It makes it possible to subscribe to a Topic while at the same time specify interest on a subset of the Topic’s data.


Note that a ContentFilteredTopic can only be used to create a DataReader, not a DataWriter.

A ContentFilteredTopic provides a relationship between a Topic, called the related topic, and some user-defined filtering properties:

  • A filter expression, which establishes a logical expression on the content of the related topic. It is similar to the WHERE clause in a SQL statement.

  • A list of expression parameters, which give values to the parameters present in the filter expression. There must be one parameter string for each parameter in the filter expression.

Note that a ContentFilteredTopic is not an Entity, and thus it has neither QoS nor listener. A DataReader created with a ContentFilteredTopic will use the QoS from the related topic. Multiple DataReaders can be created for the same ContentFilteredTopic, and changing the filter properties of a ContentFilteredTopic will affect all DataReaders using it.

Please refer to Filtering data on a Topic and Where is filtering applied: writer vs reader side for more information about how to use ContentFilteredTopic.